Ask Ethan: Is the Universe’s expansion accelerating or not?

One of the biggest surprises in all of science history came at the very end of the 20th century. For the prior ~70 years, astronomers had strived to measure the expansion rate of the Universe, hoping to discover what made up our Universe and to determine its ultimate fate. Quite unexpectedly, they discovered that the Universe wasn’t made up solely of matter and radiation, but was actually dominated by a novel, unexpected, and still poorly-understood form of energy: dark energy. Making up around 70% of the total energy density of the Universe today, it quickly became synonymous with a somewhat different phrase: the accelerated expansion of the Universe. But it turns out that the Universe’s expansion rate, which we measure as the Hubble constant (or, <a href="https://bigthink.com/starts-with-a-bang/hubble-constant/" rel="noopener ugc nofollow" target="_blank">more accurately, as the Hubble parameter</a>), isn’t accelerating or even increasing at all; it’s actually dropping. What’s the deal? That’s what Frank Kaszubowski wants to know, writing in to ask: <a href="https://medium.com/starts-with-a-bang/ask-ethan-is-the-universes-expansion-accelerating-or-not-1249ee91416f">Click Here</a>